Arthropods (Phylum Arthropoda) constitute the largest phylum of animals, and include the insects, arachnids (e.g., mites, ticks, spiders), crustaceans (crabs, lobsters, shrimp), and other similar creatures. Between 75 and 80% of all organisms on planet Earth are arthropods—over a million modern species are known, and the fossil record reaches back to the early Cambrian. However, species in the world's tropical forests remain largely undiscovered; Thomas (1990) estimated that perhaps 6 to 9 million species are yet to be discovered in this environment alone. Arthropods are common in all environments, and include symbiotic and parasitic forms. They range in size from microscopic plankton (~0.25 mm) up to forms several metres long.its class insecta bear almost 1.4-1.5 million species.

Because this group is so large, we will devote the next several chapters to the various subgroups: the subphyla and classes of arthropods.

Because of the hard exoskeleton, arthropods tend to make excellent fossils. A particularly large group of arthropods known as trilobites are known only from the fossil record: florishing in Cambrian seas and into the lower Palaeozoic. The last of the triliobites disappeared at the end of the Permian.